System shuts down almost immediately after startup

Make sure a known-good battery is fully inserted. Check battery charge and make sure that at least two LED charge indicators light up; otherwise connect the adapter. The adapter LED should light when plugged in. If not, consult the MagSafe power adapter section for further troubleshooting.

After charging for a while, if the battery does not seem to charge, or if it is charged up but quickly discharges, replace the battery. Verify with a known-good battery.

If just before the system shuts down, the sleep LED briefly comes on, check the three thermal sensor connections to the main logic board. They should be fully seated with no damage to the wiring. If any thermal sensor is damaged, replace it.

You can now recondition your old batteries at home and bring them back to 100 percent of their working condition. This guide will enable you to revive All NiCd batteries regardless of brand and battery volt. It will give you the required information on how to re-energize and revive your NiCd batteries through the RVD process, charging method and charging guidelines.